Nanoparticle compositions are useful in a variety of applications and, particularly, in applications related to the production of oil and gas, including drilling and completion fluids. Nanoparticles have unique size-dependent physical and chemical properties that are typically not encountered in their larger counterparts. Nanoparticle synthesis is particularly sensitive to reaction conditions and parameters. Further, nanoparticle production at a commercial scale faces challenges, such as the lack of homogeneity of synthesis conditions, impurity of the precursor materials, limits of mass transfer between phases, and the difficulty of achieving uniform heating and mixing. Standard industrial-scale processes, such as centrifugation and filtration, are less suitable to nanoparticle production or refinement because of their small sizes. It is therefore desirable to develop a facile, large-scale and low-cost manufacturing process to produce a variety of nanomaterials.
Typical large-scale manufacturing methods of nanoparticles are often complex and involve multiple stages, such as synthesis, purification, drying, calcination, milling and size adjustment, to produce a final product with desirable properties. A number of publications discuss methods of preparing nanoparticles and nanoscale organosols. Although many reports in the literature tackle the preparation of organosols via phase transfer or other methods, these methods are complex and inefficient. They require acidic pH, heating of the mixture to high temperatures, and large quantities of transfer agents.
One of the early reports on organosols preparation via phase transfer is by Reimers and Khalafalla: Preparing Magnetic Fluids by A Peptizing Method (Bureau of Mines, Technical Progress Report 59, September 1972). In this work, heating was required to help with dispersing the aqueous ferromagnetic fluid into the oil phase, e.g. kerosene, containing transfer agent, e.g. oleic acid. Peptization into the organic phase was accomplished by either spraying the freshly prepared aqueous magnetic nanoparticle slurry into the oil phase containing the dispersing agent, or mixing the freshly prepared slurry with the oil phase containing the transfer agent at a correct ratio while heating. Heating typically imposes challenges on scalability of the approach. Different transfer agents were tested during the work of Reimers and Khalafalla; including organics having carboxylic, hydroxyl and amino groups. There was an optimum concentration of the transfer agent, which was 6 vol % to 12 vol % for kerosene. Moreover, there existed an optimum peptizing time depending on the oleic acid content (the more the oleic acid, the more the time needed). At a given concentration of the dispersion agent, increase peptizing time reduced dispersion. The resulting method required heating, did not produce a very stable organosol independent of peptizing time, and did not yield high concentrations of nanoparticles in the organosol.
U.S. Pat. No. 6,271,269 (Chane-Ching et al.) discusses the preparation of stable organosols of metal nanoparticles via phase transfer. The process involves reacting aqueous metal salts with a base to produce a colloidal dispersion of nanoparticles, which are then contacted with an organic medium containing organic acid, e.g. oleic acid. The process is preferably carried out at a temperature range between 60-150° C. The method operates in the acidic pH range (pH <2), wherein nanoparticles possess a positive charge at the surface due to adsorbed H3O+ ions (Kukkadapu, R. K., Zachara, J. M., Fredrickson, J. K., Smith, S. C., Dohnalkova, A. C., & Russell, C. K., Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions, American Mineralogist 2003, 88, 1903-1914). Under these conditions, an immiscible organic fatty acid, such as oleic acid, is protonated and tends to form clusters to minimize the total energy of the system. This in turn can shield the reactive carboxylate group and lead to inefficient adsorption onto the nanoparticle surface requiring excess acid to achieve full coverage.
A method for preparing organic colloidal dispersion of iron nanoparticles is disclosed in U.S. Pat. No. 7,459,484 (Blanchard et al.). The process employs the steps of producing an aqueous colloidal dispersion of iron nanoparticles by reacting iron salts with a base and subsequently contacting the aqueous dispersion with an organic phase containing transfer agent, e.g. oleic acid. The method operates preferably at a temperature in the range between 60-150° C. and requires the presence of an aqueous carboxylic acid or an iron organo-complex during the precipitation step. Furthermore, the process is carried out at pH between 6.5-7.5, which is near the isoelectric charge for the nanoparticles. The lack of electrostatic repulsion can lead to nanoparticle agglomeration and destabilization of the colloidal dispersion, thus requiring the addition of carboxylic acid that provides steric stabilization.
US Patent Application No. 2013/0337998 (Irving et al.) discloses the method for production of iron oxide nanoparticle aqueous dispersions and subsequent transfer into an organic phase with the aid of a transfer agent, such as oleic acid. The process requires the presence of an aqueous carboxylic acid during the precipitation step and employs pH in the range between 4-5. The molar ratio of carboxylic acid to iron ions is greater than 2.6. Furthermore, the process uses an additional oxidant.
Despite the progress made to date in improving the nanoparticle manufacturing methods, it would be desirable to provide a simple process that employs few reagents and byproducts, is easy to scale up to industrial quantities, and yields a highly-concentrated and stable product.